A generic, derivable, haskell pretty printer.

  • GenericPretty
  • A Generic, Derivable, Haskell Pretty Printer

=============================================================================== ========================== Description ========================================

GenericPretty is a Haskell library that supports automatic derivation of pretty printing functions on user defined data types.

The form of geenrics used is based on that introduced in the paper: Magalhaes, Dijkstra, Jeuring, and Loh, A Generic Deriving Mechanism for Haskell, 3'rd ACM Symposium on Haskell, pp. 37-48, September 2010, http://dx.doi.org/10.1145/1863523.1863529. Changes from the original paper in the GHC implementation are described here: http://www.haskell.org/haskellwiki/GHC.Generics#Changes_from_the_paper.

This package requires the use of the new GHC.Generics features: http://www.haskell.org/haskellwiki/GHC.Generics present from GHC 7.2. Use of these features is indicated by the DeriveGeneric pragma or the flag -XDeriveGeneric.

Pretty printing produces values of type Text.PrettyPrint.Doc, using the Text.PrettyPrint library http://www.haskell.org/ghc/docs/latest/html/libraries/pretty-

The output provided is a pretty printed version of that provided by Prelude.show. That is, rendering the document provided by this pretty printer yields an output identical to that of Prelude.show, except for extra whitespace.

The generics mechanism works on all haskell datatypes except for constrained datatypes. That is to say, datatypes which have a context will fail. For instance, data (Eq a) => Constr a = Constr a deriving (Generic) will fail because of the (Eq a) context.

=============================================================================== ===================== Installation Instructions ===============================

The package is installed in the same way as any other package. If you have cabal, you can simply do: $ cabal update $ cabal install GenericPretty

Otherwise, the steps are: 0. Make sure you have a version of ghc >= 7.4 installed and that you can use the 'runhaskell' command from the command line.

  1. Download and unpack "GenericPretty-1.2.0.tar.gz"

  2. Move to the correct directory: $ cd GenericPretty-1.2.0

  3. Run the following haskell commands to install the library globally: $ runhaskell Setup configure $ runhaskell Setup build $ runhaskell Setup install

    The last command requires root access, so you might need to run is as: $ sudo runhaskell Setup install

If something went wrong, you can check this page for more info, look at manual installation: http://www.haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package

=============================================================================== ============================== Basic Example ==================================

Here is a haskell source file, called 'SimpleTest.hs'

{-# LANGUAGE DeriveGeneric #-}

import Text.PrettyPrint.GenericPretty

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Generic)

instance (Out a) => Out (Tree a)

tree1 :: Tree Int tree1 = Node (Node (Leaf 333333) (Leaf (-555555)))(Node (Node(Node(Leaf 888888) (Leaf 57575757))(Leaf (-14141414)))(Leaf 7777777))

main = pp tree1

The flag DeriveGeneric must be given to GHC. This can be done as above, in a 'LANGUAGE' pragma, or manually by compiling with 'ghc -XDeriveGeneric'.

As can be seen, to use the library you must simply import it, derive Generic on the custom data type by typing "deriving (Generic)" and writing an empty instance of Out.

Then you can use the pretty printing functions, such as 'pp' and 'pretty'.

Compiling and running the file is simple and gives the following result.

$ ghc SimpleTest.hs $ SimpleTest

Node (Node (Leaf 333333) (Leaf (-555555))) (Node (Node (Node (Leaf 888888) (Leaf 57575757)) (Leaf (-14141414))) (Leaf 7777777))

If we replaced the main function with 'main = ppLen 30 tree1', the result would instead be:

Node (Node (Leaf 333333) (Leaf (-555555))) (Node (Node (Node (Leaf 888888) (Leaf 57575757)) (Leaf (-14141414))) (Leaf 7777777))

In this case the output tries to remain under 30 characters/line, if possible, while always maintaining correct indentation.

=============================================================================== ================= The 'ppStyle' and 'prettyStyle' functions ===================

The 'ppStyle' function lets you further customize the output by giving a 'Style' which consists of the line length, the number of ribbons per line and the mode to use.

The 'prettyStyle' function does the same thing except it outputs a String instead of an IO() operation.

A ribbon length is the maximum length of non-indentation text per line. So if I used a line length of 80 and 2 ribbons per line than I would have a maximum of 40 non-indentation characters on any line.

The mode tells 'Pretty' how to render the result. There are 4 options:

  1. PageMode - the default rendering
  2. ZigZagMode - zig-zag cuts
  3. LeftMode - there is no indentation and no maximum line length
  4. OneLineMode - everything is put on one line

The most interesting one is the ZigZagMode. Modifying the running example we get:

{-# LANGUAGE DeriveGeneric #-}

import Text.PrettyPrint.GenericPretty import Text.PrettyPrint

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Generic)

instance (Out a) => Out (Tree a)

tree1 :: Tree Int tree1 = Node (Node (Leaf 333333) (Leaf (-555555)))(Node (Node(Node(Leaf 888888) (Leaf 57575757))(Leaf (-14141414)))(Leaf 7777777))

zigStyle :: Style zigStyle = Style {mode = ZigZagMode, lineLength = 30, ribbonsPerLine = 1.5}

main = ppStyle zigStyle tree1

We import "Text.PrettyPrint" to gain access to the "Style" functionality. Then we proceed to define the 'zigStyle' using a maximum line length of 30, 1.5 ribbonsPerLine and the ZigZagMode.

Running the program, we get:

Node (Node (Leaf 333333)

///// (Leaf (-555555))) (Node (Node (Node (Leaf 888888)

///// (Leaf 57575757)) (Leaf (-14141414))) (Leaf 7777777))

Notice that the "/" show us the direction in which the rows below have been moved (left in this case) and the number of "/"s indicate the number of characters that the rows were moved(in this case 5 characters to the left).

So the last 3 lines are 10 characters to the left compared to where they would be if we had no maximum line length.

=============================================================================== ======================= Full Customization Example ============================

While the previous approach provides us some with some options as to the format of the pretty printed result, sometimes you need even more control.

Fully customizing the pretty printed results is straightforward, as in the following example called 'CustomTest.hs'

{-# LANGUAGE DeriveGeneric #-}

import Text.PrettyPrint.GenericPretty import Text.PrettyPrint

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Generic)

instance (Out a) => Out (Tree a) where doc (Leaf a) = parens $ text "customLeaf" <+> doc a doc (Node a b) = parens $ text "customNode" \[ nest 1 (doc a) \] nest 1 (doc b) docPrec _ = doc

tree1 :: Tree Int tree1 = Node (Node (Leaf 333333) (Leaf (-555555)))(Node (Node(Node(Leaf 888888) (Leaf 57575757))(Leaf (-14141414)))(Leaf 7777777))

main = pp tree1

Here we import the library 'Text.PrettyPrint' and use it directly to define doc. We then define 'docPrec' in terms of 'doc', ignoring the precedence parameter. We could have manually defined 'docList' as well if we wanted. As it is now, 'docList' is inferred from 'doc'.

Note As opposed to the 'Show' class, 'doc' and 'docPrec' are NOT defined in term of one another. This means that even after giving a custom definition for one of them, the other will retain the default behaviour. If both are to be used, then custom definitions must be provided for both.

For instance, in the above example, if we had ommited 'docPrec's definition, the result would have been identical to that of 'SimpleTest.hs'.

By running the above custom definition we get a tree with little indentation:

(customNode (customNode (customLeaf 333333) (customLeaf -555555)) (customNode (customNode (customNode (customLeaf 888888) (customLeaf 57575757)) (customLeaf -14141414)) (customLeaf 7777777)))

The details of the syntax used above can be found in the Text.PrettyPrint library: http://www.haskell.org/ghc/docs/latest/html/libraries/pretty-

=============================================================================== ========================= Further Info ========================================

The above 'Tree' examples can be found in 'TestSuite/SimpleTest.hs', 'TestSuite.ZigZagTest.hs' and 'TestSuite/CustomTest.hs'. More involved examples integrated with QuickCheck can be found in 'TestSuite/Tests.hs'.

Further information can be found in the API: http://hackage.haskell.org/packages/archive/GenericPretty/latest/doc/html/Text-PrettyPrint-GenericPretty.html and in the source code itself.

=============================================================================== ============================ Contact ==========================================

Please send any questions/suggestions/issues to: Razvan Ranca - ranca.razvan@gmail.com